1. Field of the Invention
The field of the invention is respiratory face masks, methods and apparatus for determining air tight fit of the mask to the face of the wearer.
2. Description of the Related Art
Respirators, also occasionally referred to as face masks or gas masks, are used to protect personnel from breathing in contaminants while exposed to a contaminated environment. Respirators fall into two basic classes, the first class being a supplied air respirator in which a flexible hose connects a supply of clean air to the respirator, and the second class where the respirator draws air from a surrounding contaminated environment. The latter class is the most widely used of all respirators and respirators of this class generally are constructed to cover the wearer's nose and mouth with a flexible rubber mask which is held in place with an air tight relationship to the face as much as possible through the use of one or more elastic holding straps which encircle the wearer's head.
Respirators, in the most part, are constructed of various elements comprising firstly a facepiece which may be constructed of rubber or silicone rubber and is that part which covers the nose and mouth of the wearer. The facepiece, which is differently sized and formed to fit the face, is held in place by means of the aforementioned rubber or elastic head bands which attach, by means of snaps, to the facepiece and surrounds the head in one or more loops.
In the usual respirator of the second class, three apertures are formed in the facepiece, two on opposite sides and one in the lower center area. The two apertures on opposite sides are designed to receive the inhalation filter cartridges which are the means by which contaminants are filtered from the environmental air and provides the path for air pulled into the facepiece by the negative pressure created interiorly by the person inhaling. These inhalation filter cartridges, which appear to be extensions of the wearer's cheeks, are built-up devices having cartridge adaptors, inhalation valve flaps, filters of different types, perforated filter covers, gaskets, and the like. In addition, innerchangeable cartridges are available which combine the filter and filter cover into a single cartridge which is screwed on to threads formed on the cartridge adaptor. The cartridge adaptor is in an air-sealed relationship to the facepiece. In the lower center portion of the facepiece is the exhalation valve which opens during the time the wearer is exhaling, i.e., when there is an over-pressure interiorly to the facepiece relative to the environment, and the exhalation valve closes when the wearer inhales, i.e., there is a negative pressure interiorly to the facepiece relative to the environment. In addition, it is common also to place oppositely operating, but similar type valves in the inhalation filter cartridges, i.e., upon an over pressure interiorly to the facepiece, the valve closes.
By innerchange of different types of filter elements, a respirator may be specifically designed for a particular environment. For example, activated charcoal acts as a scrubber for gases whereas felt, cloth, or paper may be utilized in a paint aerosol environment.
As can well be imagined, of primary concern is the fit of the respirator against the face of the wearer insomuch much as if there is not an air tight fit, the environment will be drawn into the face mask between the wearer's face and the respirator upon inhalation, and thus the purpose of the respirator is defeated or at least in part. Various tests and methods have been devised to determine a "fit factor" for a respirator as applied to a certain person and the way the test is designed, the higher the number the better the fit. Thus, the fit factor is a ratio of the contamination level outside the mask divided by the contamination level inside the mask; or alternatively the ratio of total (purified+contaminated) air inspired divided by contaminated air inspired. For example, if a person breathes in air at a rate of 35 liters/minute and it has been determined that 350 milliliters/minute did not enter through the purifying inhalation filter cartridges, the fit factor is a ratio of 35 l./minute.div.0.35 l./minute=100.
The most common method used today of determining fit factor for respirators is to place a person in an environment with a known concentration of contamination, collect air from the mask interior, and then determine the concentration of the contaminant in such collected air. Air borne contaminants which are commonly used in tests of these types are di-octal phthalate, commonly called DOP, corn oil, and sodium chloride salt fogs. The techniques by which monodispersed contaminant particles are precisely generated and uniformly dispersed in air for these tests are generally rather complicated.
Another major problem in evaluating respirators through today's methods is the method by which the concentration of the air borne contaminant, more commonly called aerosols, is measured. One of the most popular methods used today is to measure concentration through light scattering techniques, i.e., shining a light through a known volume of the captured contaminants and then determining through photometric cells and light scattered which is related then to concentration. However, this method has problems as in many cases, the measuring equipment lies some distance away from the party under test (usually ouside a sealed chamber) and hoses used to convey the breathed air with contaminants may be porous or partially porous to the particular contaminant or may adsorb the contaminant.
As may well be imagined, since wearer's faces are differently shaped and sized, obviously one respirator is not going to fit all people. Accordingly, the companies manufacture different sizes. Nevertheless, from the very fact that there are different sized available in most respirators, attempts to fit the respirator to one particular person means that there is still a compromise. In addition, the rate of contaminant leakage changes as the wearer breathes at different rates and volumes because of different work rates. The fit factor determined for a wearer in a resting condition may not adequately describe the fit factor achieved with the same respirator under more vigorous work conditions.
Consequently, missing from the field of respirator fit data is how well respirators fit a person and what degree of protection is afforded a wearer who wears the mask over a long period of time and under varying conditions of work.
During inhalation, or as more commonly called in the field, "inspiration", the inspiratory volume and the inspiratory flow rate, i.e., the rate of movement of air into the wearer's lungs, causes a negative pressure difference between the environment outside the mask, and the interior of the face mask. Increasing inspiratory volume and increasing inspiratory flow rate causes a greater negative pressure to be induced inside the mask during more rigorous work conditions. The varying of negative pressure interiorly to a mask simulates varying conditions of work of the wearer, and thus provides a method for determination of fit factor under the varing conditions.
In addition, because of the time, expense, and difficulty in determining fit factor for a person of a particular respirator, many workers who wear respirators day in and day out are never checked to see which respirator, of all available respirators, achieves for them the highest, and thus the safest, fit factor in order that maximum protection may be afforded.
Accordingly, it is apparent that there exists a need for method and apparatus by which the fit factor for any one mask upon an individual's face may be determined, and determined under conditions which the wearer may expect to encounter during his work day.